There are different ways of connecting the Volvo Cars app to the car.
Before selling your car, make sure to remove the car in the Volvo Cars app. When you do this, all related data will be deleted from both the app and the car. It will cause inconvenience for the next owner of the car if you uninstall the Volvo Cars app in your phone without removing the car in the app.
A symbol visible in the top left corner of the centre display shows the car's connectivity status. If there is a problem, there are a few things you can try to restore the connectivity.